The invention relates to a compression moulding manufacturing method for plastic parts with a neck in which an orifice is formed. These parts are usually receptacles or part of receptacles. The invention is more particularly applicable to conditions for high production rate manufacturing of moulded objects, that usually have an axisymmetric neck delimiting an approximately circular orifice, for example heads of flexible plastic tubes comprising a neck provided with a dispensing orifice and a shoulder connecting the said neck to a cylindrical flexible skirt. We will use these flexible tube heads to illustrate this invention.
In general, a flexible tube is made by assembly of two parts made separately; a cylindrical flexible skirt with a given length (typically 3 to 5 times the diameter) and a head comprising a neck provided with a dispensing orifice and a shoulder connecting the said neck to the cylindrical skirt. The plastic head may be moulded separately and then welded onto one end of the skirt, but the head is advantageously moulded and welded autogenously to the skirt using either an injection moulding technique (FR 1 069 414) or a compression moulding technique for an extruded blank (FR 1 324 471).
In both of these two techniques, the skirt is fitted around a punch, one of the two ends projecting slightly beyond the end of the punch, the said end of the punch acting as a mould for making the inner surface of the tube head (inner surface of the shoulder and the neck). A die is used in these two techniques, that comes into contact with the end of the punch, the cavity of this die defining the outer surface of the shoulder and the neck. The main difference between these methods is that in the first case, these tools are firstly pushed firmly into contact with each other before injection of the plastic material, and in the second case compression of an extruded blank is triggered as they move towards each other.
In French application No. 0103706 deposited on 19 Mar. 2001, the applicant indicated that a significant increase in production rates (for example above 250-300 units per minute) could be achieved using the compression moulding technique. In the context of this French application No. 0103706, the applicant presented a workshop for making flexible tubes in which tube heads were made by compression moulding using continuously moved tools, which provided a means of obtaining significantly higher production rates under economically acceptable conditions.
In compression moulding, production of the blank and its placement in the moulding tool creates specific problems for which solutions have been described in a large number of patents. But these problems are exacerbated when it is envisaged to use tools that move continuously and solutions proposed in the past are not suitable for this new constraint.
French application FR 1 324 471 (Karl Mägerle) describes a compression moulding method for tube heads in which the lower mould is composed of the end of a mandrel and the end of a skirt that is fitted around this mandrel, the end of the said skirt projecting beyond the end of the said mandrel; the space delimited by the end of the mandrel and the projecting part of the skirt is fed by injecting plastic material through several uniformly distributed orifices within a nozzle; the plastic material is distributed around a counter punch formed at the end of the mandrel and used to mould the inner part of the neck. Once the necessary quantity of plastic material has been injected, the nozzle is removed, the parts of the top mould are brought towards each other by radial displacement, and the plastic material is then compressed by bringing the lower mould towards the upper mould. The jets are uniformly distributed around the circumference and the material thus poured is distributed approximately uniformly around the circumference before compression is applied. The result is then an approximately uniform thickness around the dispensing orifice.
Applications FR 2 460 772 (Karl Mägerle) and U.S. Pat. No. 4,943,405 (AISA) use the idea of compressing the plastic material when it is already distributed approximately regularly circumferentially. These applications propose a toroidal extruded blank, that is fitted around a central protuberance connected to one of the mobile parts of the tool. Fitting a toroidal blank around a protuberance authorises bringing the two parts of the moulding tool located at the dispensing orifice into contact with each other before the plastic material of the compressed blank reaches this zone. More precisely, the air gap between these two tool parts is so small that no viscous flow of the plastic material can occur. Thus, with a toroidal blank, it is easier and more direct to obtain a neck provided with an orifice with a clean edge.
In FR 2 460 772, the toroidal blank is extruded using an extruding machine with a ring die opened under the control of a valve. This valve may or may not close off the annular flow of the plastic material depending on its position, and its displacement controls the size of the toroidal blank thus obtained. The use of toroidal blanks obtained by discontinuous extrusion of plastic material controlled using a valve is therefore the only known way in prior art of efficiently and directly obtaining a neck with an orifice provided with a clean edge, directly by compression moulding. However, this type of technique is not very precise and it cannot give good reproducibility of the weight of a toroidal blank, which complicates compression moulding conditions, for example compression moulding does not have the same flexibility as injection moulding, for which all surplus material can easily be evacuated.
Moreover, the toroidal blank is cooled fairly quickly by conduction in the tooling. Since the contact surface is not uniformly distributed, cooling is heterogeneous and a large proportion of the advantage provided by the toroidal geometry of the blank, namely good distribution of the material before compression, is lost. Many solutions have been proposed like that proposed in WO96/09151 (Karl Mägerie Lizenz) to reduce the amplitude and heterogeneity of cooling of the blank before compression, but they require the introduction of additional tooling elements (for example the auxiliary support sliding around the central protuberance described in WO96/09151) and means of controlling their displacement. This type of sophistication makes this type of tool economically unattractive or even prohibitive if it is required to make the mobile parts of the tooling follow a continuous overall movement.
Finally, production of the toroidal blank and its placement in the air gap between the punch and the die introduce serious difficulties when the tools are moved with continuous dynamic movements, since extrusion and injection are not suitable for continuous movement of tools, and transfer means have to be provided capable of either displacing the extrusion means themselves so that the blank can be obtained, or the toroidal blank obtained “statically” has to be recovered and placed in the air gap of the compression tools that move continuously, without deforming it too seriously.
Application EP 0 841 258 describes a compression moulding manufacturing method for plastic inserts that are introduced in proportioning caps. These inserts are provided with a cylindrical dispensing spout and have a wall closing off the dispensing orifice, which seems to indicate that the blank used is not necessarily toroidal, that it can be solid making it easier to make, and there are fewer difficulties in depositing it in the air gap of the tooling. But this wall must be removed after the insert has been formed by cutting it out using a cutting tool. This cutting operation involves a series of additional steps, and although this is possible for fairly small inserts, it is difficult to transpose it to the use of compression moulding for shaping the tube head with autogenous welding of the head onto the skirt, since the tooling involved is larger and more complex. The problem becomes even more complicated if they are to move along a continuous overall movement.
Therefore, the applicant tried to find a method for the use of compression moulding for manufacturing plastic parts provided with a neck with an orifice, that is not affected by the problems mentioned above and that can consequently be done easily using continuously moving tools.